The "temporary custody" is referring to the act of holding the offender. I believe it's a way to distinguish between actually arresting someone (i.e. an adult) and temporarily incarcerating someone in jail (i.e. a minor) until it can be determined where they should go.

Also, the act of selling a child into prostitution is still a crime. If some parent tries to sell their 15-year old kid into prostitution, the bill is saying that the parent would be incarcerated and charged, while the kid would not. Previously, everyone involved, no matter what the age, would have been arrested and jailed. The bill is basically just splitting the punishment for prostitution along age lines, which is how most laws currently handle punishment.

It would be better if underage kids who were caught in these kinds of situation could get some therapy, and a better place to go, but sadly, our society is light years away from those kinds of solutions.